Front stage upgrade, amp recommendation?

I have a 2009 Accord coupe with a stock system. I recently installed a 12" Type-R in a sealed box powered by a Alpine MRP-M500 mono amp fed by the speaker level outputs of the rear speakers.

I plan on upgrading my front stage with Polk Audio DB6501 components, should I ditch the mono amp and upgrade to a 5 channel or just add a separate amp to power the front stage? (Budget < $300 for the amp).

I will also need a line output converter, thoughts on the AudioControl LC2i?

Any advice is appreciated.